In my copending application Ser. No. 359,474 there is described a sheet of organic material having a creped or ridged formation on one surface thereof, said surface being coated with a film of thermoplastic resin conforming to the surface of the sheet of organic material wherein the creped or ridged formation forms a series of valleys or channels running transversely across the sheet, whereby entrapped air and other gases within a mold can be released to the atmosphere when the sheet is placed inside the cover of the mold with the creped or ridged surface in contact with the contents of the mold. If a foamed plastic object is produced in the mold, it fills the mold and conforms to the shape thereof. Air and other gases within the mold can escape via the furrows or grooves in the sheet, thus allowing the foamed object to completely fill the mold and avoid imperfections due to bubbles entrapped therein.
Prior attempts to allow for the lateral escape of gas under the mold cover have been directed to providing intersecting channels in the mold cover itself (see Einhorn U.S. Pat. No. 3,163,687). As explained in the Einhorn patent, channels in the mold cover which extend to the periphery of the cover have caused the additional problems of foam breakdown which creates hard horny regions where the foam cells have collapsed. Einhorn, for example, employs gas venting in the cover to avoid this problem but does not provide a way to allow for lateral escape of gas to the mold periphery without creating hard horny regions in the foamed product.
Mold cover holes are less desirable as a means of venting entrapped gas from the mold since foam may expand into the holes and create a non-uniform surface. However, until applicant's invention, no one has solved the problem of providing for the lateral escape of gas from the inside of a mold when foaming a foamable plastic material and at the same time avoid the problems of a non-uniform surface and foam cell breakdown.
Applicant has found that a flexible mold cover liner of specific dimensions allows for the lateral escape of gas from under a mold cover without creating the above-mentioned problems. The liner can be described as a series of ridges and valleys or furrows wherein the distance between adjacent ridges or between furrows or channels is from 0.3 mm. to 4 mm. and the depth of each groove or furrow is 0.1 mm. to 1.0 mm. These measurements are critical in achieving the lateral escape of gas without the adverse effects of foam cell breakdown and a non-uniform surface as explained above.
When paper is used, it is preferable to have a series of shorter ridges so that adjacent grooves or furrows are interconnected. This can be done by creping the paper, as known in the paper art as set forth below, to provide a series of ridges having a length of from 1.0 mm. to 30 mm., preferably 2.0 mm. to 22 mm. with a preferred average ridge length of about 7.0 mm.
Preferably, the distance between adjacent ridges or between furrows in applicant's mold liner is from 0.6 mm. to 1.2 mm. with an average distance of about 0.865 mm. The depth of each groove or furrow is preferably from 0.13 mm. to 0.38 mm. with an average depth of about 0.258 mm.
This system of ridges and valleys drains must end with a clear opening at the edge of the sheet disregarding how the sheet is cut. In other words, each drain or channel must have two open ends, one at each edge of the sheet. Further, those drains or channels preferably should not run straight because gases could escape too freely without the ability of maintaining the certain pressure needed for perfect foam production. Deviation to left and right in a zig-zag pattern is preferred.
While other materials can be used, paper is preferred because of the low price and ease in forming a system of ridges and valleys. Such system can be obtained for instance by embossing but in this case, the height of ripples or depth of valleys is rather limited.
Another method is creping, especially dry creping, so-called Micrexing, described in the Walton U.S. Pat. Nos. 2,761,490, 2,915,109, 3,066,046, 3,220,056, 3,220,057 and 3,260,778. This method allows a prior application of a releasing agent such as polyethylene and/or silicone.
The present invention relates to an improvement thereon wherein the film of flexible resin has a decorative or graphic design applied thereon, which design is transferred to the foamed plastic object in the mold, thus producing a three-dimensional plastic object with a graphic or ornamental design on the surface adjacent the cover of the mold.
In accordance with this invention a sheet of rippled or creped paper or similar organic sheet material coated with a thin layer of silicone resin conforming to the rippled or creped surface of the sheet material and then coated with a layer of a thermoplastic resin is used as the base material for printing or decorative design applied to the surface of the thermoplastic resin film. As disclosed in my U.S. Pat. No. 3,533,899 of Oct. 13, 1970, rippled or creped paper and similar organic material are particularly desirable for printing stock because the printing thereon is especially sharp and clear and the coverage is excellent. The printed material is uniformly darker and denser than on similar paper without the creped or rippled surface. In addition, these products are easier to cut and trim than conventional adhesive coated paper products. Therefore, when printing is applied to the thermoplastic resin, especially good characters and designs are formed.